The goal of this Phase II STTR application is to design, construct, calibrate, and test an engineering prototype of a photoacoustic imaging system capable of mapping microwave energy absorption (433 MHz) in the human breast. The detector array, associated preamplifiers, scanning mechanism and prone, patient-positioning table will be fabricated and evaluated at the Indiana University Medical Center. The data acquisition hardware and software, image reconstruction and display, and system integration will be carried out by OptoSonics, Inc. This Phase II effort is a direct outgrowth of our successfully completed Phase I research, which demonstrated that we can make accurate three- dimensional images of energy deposition patterns in breast-mimicking phantoms at both infrared (1.064 microns) and microwave (433MHz) energies. The specifications for this engineering prototype are based directly on this preliminary work. We anticipate that a full two years will be required to develop this prototype imaging system to point where clinical testing will be feasible. Due to financial constraints of the STTR program, actual clinical testing will not begin until Phase III. PROPOSED COMMERCIAL APPLICATION: X-ray mammography is the present "gold standard" for detecting occult breast cancer in women. The proposed methodology has the potential to displace x-ray mammography from this role if high spatial and contrast resolution can be demonstrated using cost-effective instrumentation. If this turns out to be the case, annual sales in excess of $100 million are possible.